There isn't really any "plot"; the main character is the titular Mr. Brown, who has not only the ability to make a great many noises, but the will to go around doing so, calmly and seriously!
What kid doesn't love the idea of a grown-up making silly animal noises? Many will join in the fun as you read it with them. This means learn to read takes place without boredom!
As with so many of Seuss's works, there is some deep pedagogical stuff going on beneath the playful surface. Take the word "Moo" in the title. Children who know that cows go "moo" don't have to be told that two o's together make an "oooooo" sound; many will see the cow, see the letter M followed by some letters, and unconsciously figure out that this is the sound "Moo"! You can ignore all that if you like. It's a fun book that many children will enjoy having re-read to them until they can read it themselves, and what could be better?